1. Field of the Invention
The present invention relates to a diffusion tube used for a diffusion process in the fabrication of semiconductor devices, a dopant source used for a diffusion process and a diffusion method using the diffusion tube and the dopant source.
2. Description of the Related Art
Semiconductors are formed when the electrical properties of certain materials or alloys are permanently modified by the introduction of impurities, in a process known as doping. Diffusion is a phenomenon where atoms move from a high concentration region to a low concentration region. The diffusion phenomenon occurs due to a gradient of concentration, and continues until a uniform concentration is achieved. The diffusion speed varies in accordance with the temperature of the atoms being diffused. The diffusion phenomenon explains the movement of impurity atoms within a chemical semiconductor.
A semiconductor device is normally fabricated by repeatedly performing a diffusion process, an ion implantation process, a photolithography process and an etch process periodically. The diffusion process is performed in order to form a layer on a semiconductor substrate or to diffuse implanted ions into a semiconductor substrate using the diffusion phenomenon. FIG. 1 illustrates a conventional ampoule used as a diffusion tube 1 in the diffusion process of the prior art. Referring to FIG. 1, the diffusion tube 1 is a conventional ampoule formed of a quartz tube, and is used when performing a conventional diffusion process using a vacuum-sealed ampoule method of the prior art. In the vacuum-sealed ampoule method, the diffusion process is performed by disposing a dopant source 3 into the diffusion tube 1; disposing a diffusion target 2 which is to be diffused on, typically a semiconductor substrate, into the diffusion tube 1; sealing the diffusion tube 1; and heating the diffusion tube 1.
However, certain types of semiconductor devices require an intense diffusion of impurities. For example, in the fabrication of a vertical cavity surface emitting laser (“VCSEL”), impurities must diffuse sufficiently deep since the mirror stack is very thick, which can range from several micrometers (μm) to tens of micrometers (μm). However, conventional diffusion processes which diffuse impurities to the sufficient depths are extremely time consuming. Table 1 illustrates experiment data achieved when zinc (Zn) impurities are diffused into a gallium arsenide (“GaAs”) substrate using a conventional diffusion method.
TABLE 1diffusiontemperaturediffusiondopant(degreesdiffusion speedelectricalmethodsourceCelsius)(micrometer/hour)characteristicsreferencesRTAZn-silicate650-7502.0 μm/hrJ. Appl.Phys.69(3) (1991),p. 1359.furnaceZnAs26000.2 μm/hrp = 1 × 1020 cm−3J. Appl.(vacuum-Phys.sealed)74(9) (1993),p. 5493.furnaceZnAs26500.5 μm/hrJ. Appl.(vacuum-Phys. 63(7)sealed)(1988), p.2454.
Referring to Table 1, the dopant source 3 in the conventional diffusion method is a zinc-silicate thin film, and a gallium arsenide chemical powder. Using the vacuum-sealed method, the diffusion speed was less than 1 micrometer per hour (μm/hr). When using a rapid thermal annealing (“RTA”) furnace, the diffusion speed was 2.0 micrometers per hour (μm/hr) at a temperature of 650 degrees Celsius. The RTA furnace, however, exhibits some problems such as vaporization of arsenic (As) on a GaAs substrate at high temperatures and the like; thereby failing to provide good diffusion results as compared to the vacuum-sealed method. Furthermore, conventional diffusion methods have many disadvantages, such as low diffusion speeds, which increase the fabrication time and the total cost for fabrication of semiconductor devices.